At cell fate decision checkpoints, a precise series of events occurs to establish developmentally appropriate gene expression profiles. At the molecular level, lineage-determinant transcription factors are required for the simultaneous activation and repression of genes that define the fate of a cell. Currently, it is unclear how these factors mechanistically achieve this precise control on a global level. In the immune system, na[unreadable]ve CD4+ T helper cells begin with the potential to become a number of phenotypically distinct lineages, with key transcription factors committing them to the defined fate that is appropriate for the pathogenic insult. The T-box transcription factor T-bet is responsible for the differentiation of the Th1 cell lineage. Previous work has shown that T-bet positively regulates the effector cytokines and chemokine receptors that are the prototypic genes in Th1 cellular differentiation. To activate the select target genes that have been studied to date, T-bet participates in at least three physically separable activities: 1) H3K27-demethylation, 2) H3K4-methylation, and 3) transactivation events that occur independent from the chromatin environment. It is currently unclear, however, whether these separable functional activities are required at all target promoters, or rather they are selectively utilized in a context-specific manner. In addition, the mechanism by which T-bet represses the gene expression profiles for the alternative helper T cell lineages and whether T-bet[unreadable]s ability to negatively regulate these target genes requires the same or distinct activities is unknown. We will examine these questions on both a global and select target gene level. To this end, we will utilize mutant T-bet constructs that are deficient in each of the defined functional activities and assess their ability to regulate the global T-bet-dependent gene expression patterns in Th1 cell differentiation. We also will determine the key protein interactions that define T-bet[unreadable]s distinct regulatory activities at both the level of physical and/or functional interactions. We will utilize this knowledge to model T-bet-dependent target gene networks that are based upon the mode of regulation as well as the phenotypic outcome. Together, these studies will allow us to define the mechanisms that are needed for the precise regulation of Th1 differentiation. Understanding the exact role that T-bet plays in immune cell differentiation is an important topic of research because the dysregulation of T-bet activity has been associated with a number of autoimmune and infectious disease states including ulcerative colitis, cancer metastasis, and murine models of multiple sclerosis. The studies proposed here will provide the base knowledge necessary for the targeted therapeutic manipulation of dysregulated T-bet activity.